This invention concerns filters of a type including a movable filter belt disposed within a pressurized filter chamber during filtration of a liquid in the chamber, the filter belt advanced out of the chamber after a filter cycle to enable removal of the solids or "cake" accumulating on the top of the belt over the course of the cycle.
The present inventor has heretofore been granted U.S. Pat. Nos. 4,430,231 and 4,568,460 on this type of filter, featuring a filter housing of fixed construction having elongated openings at either end accommodating the entrance and exit of the filter belt, with inflatable hose seals used to establish a seal above to withstand the relatively high pressure existing in the filter chamber during a filter cycle.
Prior filters of this type were constructed with separable housing sections which were clamped against the filter on four sides to establish sealing of the filter chamber.
In the patented filter, the side edges of the filter belt had to be sealed to prevent bypass flow around the lateral edges of the belt during filtering. The filter belt rests on a perforated plate having solid side borders against which the filter belt was pressed by the chamber pressure, the filter belt having impervious edges to effect this result.
In the prior filters of this type the usual practice was to stack a number of filters vertically, and a continuous belt was circulated through the chamber of each filter in zig zag fashion, with a complex guidance and drive mechanism required, and significant down time involved in changing a belt.
Accordingly, U.S. Pat. No. 4,568,460 discloses and claims an independent belt arrangement for each filter unit, in which upper and lower segments of an endless belt overlie the perforated plate and liquid flow passes through both segments. The belt is advanced after each filter cycle to transport the accumulated solids out of the filter chamber for disposal preparatory to the next filter cycle.
This design is a simplified arrangement which represents a substantial advance over the prior single belt designs, but nonetheless has certain drawbacks.
Firstly, the belt design requires overlapping segments of different widths to achieve end sealing, increasing the cost of the filter belt. The doubling of the filter belt layers increases the wearout rate, necessitating more frequent belt replacement.
The double overlapped edges also reduces slightly the width available for filtering action.
All of the endless filter belt designs require careful alignment and/or special devices to insure proper tracking of the filter belt over the support and drive rollers. Mistracking is aggravated when uneven loading of the belt occurs, as when solids accumulate more on one or the other side of the belt.
The cycle time is significantly extended in the endless belt arrangements by the need to readvance the same section of the filter belt back into position in the chamber after each movement of the belt out of the chamber at the end of the cycle.